1) Field of the Invention
This invention relates to a process for treating semiconductor wafers for producing Si-VLSI or compound semiconductor devices of high performance.
2) Prior Art
Generally, in producing ultra LSI, devices ultra-pure water is used in the surface cleaning process for the semiconductor wafers. The ultra-pure water is so low in content of impurity ions that the specific resistivity is not only higher than 18 M.OMEGA.cm, but also the water is completely free of substances, which seem detrimental to preparation of the devices, such as particulate matter, organic matter, bacteria, dissolved oxygen, etc. After cleaning with the ultra-pure water, the semiconductor wafers are transferred through the atmosphere in a clean room to a wafer drier, and dried. In a step of removing oxide films from the surfaces of semiconductor wafers by dipping the wafers in an etching solution such as dilute hydrofluoric acid, etc., the etched wafers are dipped in a water washing tank successive to the etching tank right after the etching, and the etching solution remaining on the surfaces of the wafers are is diluted and removed by ultra-pure water supplied thereto.
Hydrofluoric acid having an ultra-high purity, i.e. particularly contents of impurities being reduced to a possible minimum limit for electronic industry purposes, is generally used for the removal of oxide films from the surfaces of semiconductor wafers, where impurities and particles are given a particularly keen attention and thus hydrofluoric acid for use in the etching is subjected to ultra-fine filtration, etc. Hydrofluoric acid is inevitably so expensive that it is repeatedly used by storing it in the etching tank.
However, in the conventional method an ultra-pure water film remains on the wafer surface, as attached thereto, when the wafer is picked up from the ultra-pure water to transfer it to the wafer drier after the cleaning with the ultra-pure water, and thus it is necessary to transfer the wafer through the atmosphere. It has been found that the impurities existing in the atmosphere dissolve into the ultra-pure water film remaining on the wafer surface, as attached thereto, to contaminate the wafer surface. That is, in spite of the cleaning with ultra-pure water, that is, water of very high purity, the above-mentioned phenomenon is equal to attachment of atmospherically contaminated water to the wafer surface in the course of transfer of the wafer to the drying step after cleaning with ultra-pure water, and this is a source of wafer surface contamination. For example, in case of high density VLSI of 64 Mbi or more, the gate oxide film has a thickness of not more than about 100 .ANG. and the presence of a trace amount of contamination during the oxidation is a cause for deterioration of device characteristics such as deterioration of gate oxide film, etc., or a decrease in fabrication yield.
Generally, a clean room atmosphere freed from particles to a very high degree is used, but this atmosphere contains gas molecules of carbon dioxide gas exhaled from workers or hydrogen fluoride, nitrogen oxide, etc. emitted from hydrofluoric acid, nitric acid, etc. used in the clean room. These gas molecules chemically react with metallic parts used in an air-conditioner, etc., or with boron, etc. contained in Pyrex materials on the material surface together with water vapor in the atmosphere to form such molecules as metal salts, which contaminate the clean room atmosphere. Generally, these impurity molecules in the clean room atmosphere are highly water-soluble, and when the wafers are transferred through the clean room atmosphere while retaining water films as attached to the wafer surfaces after the cleaning with ultra-pure water, the impurity molecules dissolve into the wafer films from the clean room atmosphere to cause a surface contamination.
The surface contamination gives rise to various problems. For example, the specific resistance of ultra-pure water is lowered to about 12 M.OMEGA.cm by mere contact of ultra-pure water with the impurities-contaminated atmosphere, thereby instantaneously absorbing carbon dioxide molecules into the ultra-pure water from the atmosphere. Experimental results of Si epitaxial growth show that carbon contamination, which seems to be due to adsorption of carbon dioxide molecules, has been detected on the wafer surfaces in transfer from the cleaning step to the drying step to increase crystal defects of expitaxially grown film [Y. Hirofuji and Naoto Matsumoto; J. Vac. Sci. Technol. B5 (1), pp. 10-14, Jan/Feb (1987)].
Experimental results of polycrystalline Si laser recrystallization show that boron compounds have been deposited on the wafer surfaces from the atmosphere in the cleaning step before polycrystal deposition and after the drying step, and recrystallized semiconductor thin films have turned p-type, making it difficult to control the threshold voltage of MOS transistor formed on the crystal [Y. Itoh et al; Prelecture Summaries of No. 37 Meeting of Oyobutsuri Kankei Rengo-gakakai (Society of Applied Physics-related Sciences), Part 2, page 620 (1990) 3a-SC-4].
Furthermore, particularly when a wafer surface-treating solution destined for repeated use such as dilute hydrofluoric acid is preserved in the atmosphere, the impurities dissolve into the solution from the atmosphere, as mentioned before, and the impurity concentration of the solution increases with time. When the wafer surfaces are treated with such a treating solution, impurities are adsorbed also on the wafer surfaces in accordance with the impurity concentration of the solution. Particularly in case of dilute hydrofluoric acid, the wafer surfaces in an oxide film-free state are highly active in the solution, and thus special attention must be paid to the adsorption of impurities. Even if the wafer surfaces are hydrophobic, the surfaces of wafers withdrawn from the etching solution are very liable to adsorb the impurities from the atmosphere, and thus wafer surface contamination from the atmosphere takes place until the wafers are dipped in a water washing tank filled with ultra-pure water to dilute and remove the etching solution remaining on the wafer surfaces.
Such phenomena occur not only with dilute hydrofluoric acid or ultra-pure water, but also with such cleaning solutions as organic solvents, etc., because the impurities of the atmosphere likewise dissolve into the organic solvents, etc., causing a wafer surface contamination. The surface contamination has a larger influence upon the characteristics of semiconductor devices and also cause a decrease in the yield in the production process. Thus, the surface contamination is a very serious problem against recent requirements of semiconductor devices, such as higher performance, higher density and smaller size.